DVPLICAT 



m 
APR 16 19^^ 



Department of Agricultural Investigations 
American Smelting and Refining Company 



Sterelity in Oats Caused by 
Grass Thrips 



^ SvBy 
Piij(«b'GARA, 

Chief in Charge 



February 1 , 1915 



C3^b0« 



0^ 



^^ 



Sterelity in Oats Caused by Grass Thrips 



During the season o£ 1914 our attention was 
frequently directed to a condition whicti pre- 
vailed in practically all of the oat fields in the 
Salt Lake valley, namely, a blighting of the ears 
or spikelets. About the time that the oat plants 
were heading it was noted that many of the 
pikelets were smaller than normal and were 
whitish and shrivelled. Upon examination it was 
found that these whitish spikelets had no kernels 
in them, there being nothing left but the empty 
husks or glumes; frequently more than fifty per 
cent of the spikelets were found to be injured 
and contained no kernels. Invariably, the cause 
of this condition was not understood by the 
farmers of the valley. 




A — Head of oats seriously injured by grass 
thrips (Anaphothrips striatus). Note the small 
white spikelets in which there are no kernels. 

D — Head of oats in which only a few of the 
lower spikelets have been injured. Specimens 
taken in Salt Lake valley by W. W. Jones. (Or- 
iginal.) 



Popular Name for Injury 

This type of injury to oats has been known 
for a good many years, and has been frequently 
referred to as "White-top" and Silver-top" <by 
reason of the peculiar whitened appearance of 
the injured spikelets. We have records of injury 
of this sort from many portions of the United 
States, Canada and Europe. The cause of the 
injury is due to a very minute insect known as 
the grass thrips, Anaphothrips striatus.j This 
insect is so small and so active in its move- 
ments that the trained observer will often falT 
to find it, while the farmer would never see it, 
even though he were to look for it. This insect 
averages about one-twentieth of an inch in length 
and being of a light yellow color is rarely ever 
noted by the casual observer. 

Life History of the Insect 

The length of the female insect, as stated 
above, averages about one-twentieth of an inch. 
The color is yellow, with more or less dusky or 
brownish shading in some of its parts. Male 
insects have been found infrequently and, there- 
fore are much fewer in number than the female. 
The eggs are bean-shaped and are very minute, 
being slightly more than one one-hundredth of 
an inch in length. The adult insects hibernate, 
that is, live over winter, in places where they 
have been feeding, such as the slcius of grains 
which have been cut and have died down, as 
well as under rubbish and in crevices. They are 
able to withstand very low temperatures, even 
as low as fifty-three degrees below zero Fahren- 
heit. The females become active very early in 
the spring, at least as soon as the grasses and 
grains begin to grow. Egg laying soon takes place 
and may last for a number of weeks, each fe- 
male being able to deposit from fifty to sixty 
eggs. In from ten to fifteen days, during the 
spring months, the eggs are hatched; while dur- 
ing the summer the time required is from four 
lo seven days. It requires from twelve to thirty 
days for the insect to pass from the egg stage 
to the mature or adult stage. Numerous broods, 
therefore, appear or are produced, during the 
summer season, and it has been noted in the 
Salt Lake valley that this insect is active at 
least up to the early part of December when-" 
aver green plants may be found to feed upon. 
During the past season this insect was noted 
as being active on volunteer oats as late as De- 
cemher 12th in localities where the frost had not 



U U^l tau ^10 



completely killed the plants. The insect does 
not confine its attacks to oats, but is found to 
injure rye, wheat and various grasses. It is a 
peculiarly injurious insect when it attacks tim- 
othy. 

Injury Produced 
In oats the injury is largely confined to the 
spikelets, although the leaves may also be at- 
tacked. The injured spikelets are usually more 
or less whitened or bleached in appearance and 
are shrivelled and thin. Such spikelets contain 
no kernels or grains. This injury is largely pro- 
duced before the spikelets emerge from the 
sheath or boot. The insects, both young and 
old, feed mainly within the leaf shea'.hs but 
may also he found doing Injury outside. The 
spikelets are attacked before they emerge from 
the leaf sheaths, both the young and old insects 
feeding upon them, producing complete sterility 
by sucking the ovaries and feeding upon other 
young parts of the flower. It is only when the 
spikelets emerge from the sheaths that the ex- 
tent of the injury is noted. As previously stated 
the injury to oats may reach as high as fifty per 
cent or more. From what has been noted during 
the season of 1914 in the Salt Lake valley, it is 
easy to explain the low yields in most fields. It 
may be said in passing that the insect which 
produces such severe Injury to oats belongs to 
a group or family of insects, many members of 
which cause great losses to the farmer. As an 
■^**^IftnrtSfie, it may be noted that an insect very 
closely related to the insect described above 
produces heavy losses to the pear and cherry 
growers of California. It also attacks other 
fruits in a serious degree. In California it is 
known as the most dangerous and destructive in- 
sect to certain deciduous fruits, and it is one 
of the hardest to successfully combat. 

Control Measures 
As the Grass Thrips, which has been noted 
as seriously attacking oats, feeds on other cer- 
eals as well as on grasses, it is evident that con- 
trol would be difficult. Knowing its habits, it 
is readily seen that grasses, weeds and volun- 
teer crops of grain should be destroyed in one 
way or other so as not to provide food and 
places to hibernate in fields where it is intended 
to grow oats the following year. It is recom- 
^^^ mended that deep fall plowing be practiced, and 
^^ Jhis will have its advantages outside of the mat- 
ter of insect control. It has been suggested also 
that the stubble be burned after the grain has 
been cut, but this Is a poor practice from the 
agricultural point of view. It has been noted 
that early varieties of oats are less seriously 



injured than the later ripening varieties, and if 
this proves true, advantage may be taken of 
them. Where it is desired to put in winter var- 
ieties of cereals, such as wheat or oats, it has 
been suggested that either rye or oats be sown 
in a strip around the field some two or three 
weeks before planting the winter grain. The in- 
sects are attracted to such a "trap" crop, and if 
this crop is removed before the permanent crop 
appears above the ground, large numbers of the 
insects will be destroyed. 

In closing it may be said that there is abso- 
lutely no doubt as to the cause of the injury to 
oats in the Salt Lake valley, as noted In this 
paper, or to the identity of the insect. The find- 
ings of the writer have been fully corroborated 
by no less an authority than Dr. W. E. Hinds, 
who for several years was an entomologist in 
the U. S. Department of Agriculture, and who 
is now entomologist at the Alabama Agricultural 
Experiment station. Dr. Hinds made a special 
study of this group of insects for several years, 
and is, therefore, our American authority or 
thrips. 




Illustration of Grass Thrip3 (Anaphothrips stri- 
atus) Enlarqed About 55 Diameters 

The area covered by the illustration is ap- 
proximately three thousand times the area that 
would be covered by the life size specimen. 
Note the peculiar wings, left being folded and 
the right extended. 
(After Fernald and Hinds) 



IIIIIIIIIIMIIMIIIIMI 

021 490 275 8 



completely killed the plants. The insect does 
not confine its attacks to oats, but is found to 
injure rye, wheat and various grasses. It is a 
peculiarly injurious insect when it attacks tim- 
othy. 

Injury Produced 
In oats the injury is largely confined to the 
spikelets, although the leaves may also be at- 
tacked. The injured spikelets are usually more 
or less whitened or bleached in appearance and 
are shrivelled and thin. Such spikelets contain 
no kernels or grains. This injury is largely pro- 
duced before the spikelets emerge from the 
sheath or boot. The insects, both young and 
old, feed mainly within the leaf sheaths but 
may also be found doing Injury outside. The 
spikelets are attacked before they emerge from 
the leaf sheaths, both the young and old insects 
feeding upon them, producing complete sterility 
by sucking the ovaries and feeding upon other 
young parts of the flower. It is only when the 
spikelets emerge from the sheaths that the ex- 
tent of the injury is noted. As previously stated 
the injury to oats may reach as high as fifty per 
cent or more. From what has been noted during 
the season of 1914 in the Salt Lake valley, it is 
easy to explain the low yields in most fields. It 
may be said in passing that the insect which 
produces such severe injury to oats belongs to 
a group or family of insects, many members of 
which cause great losses to the farmer. As an 
aJMje, it may be noted that an insect very 
closely related to the insect described above 
produces heavy losses to the pear and cherry 
growers of California. It also attacks other 
fruits in a serious degree. In California it is 
known as the most dangerous and destructive in- 
sect to certain deciduous fruits, and it is one 
of the hardest to successfully combat. 

Control Measures 
As the Grass Thrips, which has been noted 
as seriously attacking oats, feeds on other cer- 
eals as well as on grasses, it is evident that con- 
trol would be difficult. Knowing its habits, it 
is readily seen that grasses, weeds and volun- 
teer crops of grain should be destroyed in one 
way or other so as not to provide food and 
places to hibernate in fields where it is intended 
to grow oats the following year. It is recom- 
mended that deep fall plowing be practiced, and 
■ Jhis will have its advantages outside of the mat- 
tei of Insect control. It has been suggested also 
that the stubble be burned after the grain has 
been cut, but this is a poor practice from the 
agricultural point of view. It has been noted 
that early varieties of oats are less seriously 



injured than the later ripening varieties, and if 
this proves true, advantage may be taken of 
them. Where it is desired to put in winter var- 
ieties of cereals, such as wheat or oats, it has 
been suggested that either rye or oats be sown 
in a strip around the field some two or three 
weeks before planting the winter grain. The in- 
sects are attracted to such a "trap" crop, and if 
this crop is removed before the permanent crop 
appears above the ground, large numbers of the 
insects will be destroyed. 

In closing it may be said that there is abso- 
lutely no doubt as to the cause of the injury to 
oats in the Salt Lake valley, as noted In this 
paper, or to the identity of the insect. The find- 
ings of the writer have been fully corroborated 
by no less an authority than Dr. W. E. Hinds, 
who for several years was an entomologist in 
the U. S. Department of Agriculture, and who 
is now entomologist at the Alabama Agricultural 
Experiment station. Dr. Hinds made a special 
study of this group of insects for several years, 
and is, therefore, our American authority on 
thrips. 




Illustration of Grass Thrips (Anaphothrips strl- 
atus) Enlarged About 55 Diameters 

The area covered by the illustration is ap- 
proximately three thousand times the area that 
would be covered by the life size specimen. 
Note the peculiar wings, left being folded and 
the right extended. 
(After Fernald and Hinds) 



